"One slip and the day's ruined"

Now the Thursday biscuit, because this is where the diet actually ends, and it isn't where you think.

The biscuit isn't the end. It's a biscuit. Eighty or so calories, gone in a minute, and if that were all that happened on Thursday the diet would still be going on Friday. What ends the diet is the sentence after the biscuit. "Well, that's today gone." The write-off. And the write-off is what turns one biscuit into four, and the four into the thing at home, and Thursday into Friday's takeaway and the word "Monday".

Herman and Mack's restrained eaters, in chapter 3, did exactly this in a laboratory. Having had the preload, they ate more, where non-dieters ate less. The researchers' account was that the preload had broken the diet for the day, and a broken diet has nothing left to protect. Might as well. That's the write-off, observed in 1975, and it's the single mechanism by which a small break becomes a large one.

Notice what the write-off assumes. It assumes that the day is the unit. That a day with a biscuit in it is spoiled, the way a day with one drop of ink in a glass of water is spoiled, and the only thing to do is throw the day away and start a fresh one. But a day isn't a glass of water. It's a series of meals, and a biscuit at half past three has no bearing on dinner at seven unless you decide it has. A person who isn't dieting knows this without being told. They have a biscuit, and then they have dinner, and the two things don't touch.

So the belief in this chapter is false in a precise way. One slip doesn't ruin the day. One slip plus the write-off does, and you supply the write-off, and you can stop supplying it. That's most of chapter 20, and it's the most important practical thing in the book, so I'll give it here in one line. After the biscuit, the next meal is an ordinary meal. Not smaller to make up. Not skipped. Not a fresh start. Ordinary.

If you take that one habit and nothing else from this book, the cycle can't run, because the cycle needs the write-off to get from the first break to the blame.